1. Field of the Invention
The present invention relates to an image processing system, and more particularly, to a method of processing an image based on block units, and an apparatus therefor.
2. Description of the Related Art
In an image processing system for compressing or filtering an image, the entire image is divided into a plurality of sub-images, and the image processing is performed within each sub-image. Such image processing is called a block-based image process. The block-based image process can reduce the computation amount required to process an image, and can allow effective hardware configurations, and so has been widely used.
However, the block-based image process causes blocking artifacts due to discontinuity at the boundaries of the blocks, which have been separately processed, thereby deteriorating image quality.
For example, such blocking artifacts are generated in compressing an image with block-based discrete cosine transform used for coding still images and moving images in a digital high definition television (HDTV), the Joint Photographic Expert Group (JPEG) method and Moving Picture Expert Group (MPEG) method.
In order to remove the above blocking artifacts, the following methods have been suggested.
First, a method of removing blocking artifacts by low-pass filtering the brightness values of pixels existing at the boundaries of blocks is disclosed in article entitled xe2x80x9cNonlinear space-variant postprocessing of block coded imagesxe2x80x9d, IEEE Transactions on Acoust., Speech and Signal Processing, Vol. 34, No. 5, pp. 1258-1267, October 1986, by B. Ramamurthi and A. Gersho. However, by this method, actually the brightness values of the pixels adjacent to the boundaries of blocks are low-pass filtered as well as those of the pixels belonging to the boundaries, thereby blurring the image.
Second, there is a method of iteratively reducing spatially adaptive blocking artifacts based on regularization theory, which is disclosed in an article entitled xe2x80x9cRegularized reconstruction to reduce blocking artifacts of block discrete cosine transform compressed imagesxe2x80x9d, IEEE Trans. on Circuits and Syst. for Video Technolo. Vol. 3, No. 6, pp. 421-432, December 1993, by Y. Y. Yang, N. P. Galatsanos and A. K. Katsaggelos.
Third, a method of reducing blocking artifacts by iteratively projecting an image onto convex sets satisfying constraints that a negative brightness value of pixel is not allowed and the boundary is continuous has been suggested in articles by R. Rosenholtz and A. Zakhor (xe2x80x9cIterative procedures for reduction of blocking effects in transform image codingxe2x80x9d, IEEE Trans. on Circuits and Syst. for Video Technol, Vol. 2, No. 1, pp. 91-94, March 1992), S. J. Reeves and S. L. Eddins (Comments on xe2x80x9citerative procedures for reduction of blocking effects in transform image codingxe2x80x9d, IEEE Trans. on Circuit and Syst. for Video Technol., Vol. 3, No. 6, pp. 439-440, December 1993), and Y. Y. Yang, N. P. Galatsanos and A. K.
Katsaggelos (xe2x80x9cProjection-based spatially adaptive reconstruction of block-transform compressed imagesxe2x80x9d, IEEE Trans. on Image Processing, Vol. 4, No. 7, pp. 896-908, July 1995).
Fourth, a method of removing blocking effects by changing discrete cosine transform (DCT) coefficients by minimizing a predetermined cost function has been suggested by S. Minami and A. Zakhor (xe2x80x9cAn optimization approach for removing blocking effects in transform codingxe2x80x9d, IEEE Trans. on Circuits and Syst. for Video Technol., Vol. 5, No. 2, pp. 74-82, April 1995) and J. Jeong and B. Jeon (xe2x80x9cUse of a class of two-dimensional functions for blocking artifacts reduction in image codingxe2x80x9d, Proceedings. 1996 International Conference on Image Processing, pp. 478-481, 1995).
However, when blocking artifacts are caused by a compression and reproduction process, then the inverse process, for removing the blocking artifacts, has no intrinsic solution, thus the iterative image restoration method such as the second and third conventional methods, or the constrained optimization method such as the forth conventional method, have been used. Here, having the intrinsic solution in the inverse process means that the original image before the block-based image process is obtained by the inverse process of the block-based image process. However, it takes much time to obtain a solution which approximates the intrinsic solution, using the above second and third iterative methods. Thus, application of such methods is restricted to non-real time processes such as JPEG coding. Also, the second, third and fourth methods process even still images slowly, thereby causing restrictions in use.
A moving image compression system for compressing a moving image using the moving image compression standard, such as H.261, H.263, MPEG1, MPEG2 and MPEG4, has relatively many application fields in comparison with still image compression system. Thus, a method of processing the blocking effects of a moving image in real time is required. The moving image compression system causes the blocking effects if a bit rate is less than a predetermined threshold value. Such blocking effects occur at a rapid image changes, thereby further deteriorating the image.
In addition, as a method of expanding contrast of an image, there is a non-overlapping histogram equalization method in which the entire image is divided into uniform blocks which do not overlap each other, and then the histogram equalization is separately performed within the divided block. However, because each block is separately processed, the adjacent blocks show discontinuities, causing the blocking artifacts.
It is an objective of the present invention to provide a block-based image processing method capable of removing blocking artifacts, caused by another block-based image process, by using spatial adaptive filtering based on an image restoration theory.
It is another objective of the present invention to provide a block-based image processing apparatus for the block-based image processing method.
To achieve the first objective, there is provided a block-based image processing method for removing blocking artifacts induced in processing an image in blocks of a predetermined unit size, the method comprising the steps of: (a) dividing the processed image signal into blocks of the predetermined unit size; (b) detecting an edge direction of the image for each block divided; (c) band-pass filtering the processed image signal to obtain plurality of filtered image signals equal in number to the possible types of edge direction; (d) selecting one of the band-pass filtered image signals for each block, according to the detected edge direction; and (e) synthesizing the selected image signals for all the blocks to obtain an original signal from which the blocking artifact is removed.
To achieve the second objective, there is provided a block-based image processing apparatus for removing blocking artifacts induced by processing an image in blocks of a predetermined unit size, comprising: an edge direction detecting portion for detecting an edge direction of the processed image signal for each block; first through S-th band pass filters for filtering the processed image signal at S different predetermined bands and outputting the filtered results, wherein S is equal to the number of edge directions; a signal selector for selecting at least one of outputs of the first through S-th band pass filters in response to the detected edge direction; and a signal synthesizer for synthesizing the selected image signals for all blocks, output from the signal selector, and outputting the synthesized signal as an original image signal from which the blocking artifacts are removed.